Apparatus for the growth of single crystal ingots by the Czochralski method includes a crucible to contain a melt and a mechanism for concurrently drawing a crystal from the melt along a vertical axis at a steady rate while providing relative rotation about this axis for the growing crystal with respect to the melt. Vertical lift of the crystal ingot is effectuated from the "pulling head" portion of the apparatus which must maintain mechanical alignment of the axis of rotation with the vertical pulling axis while providing case of access for maintainence and rapid turnaround time in a production context. Moreover, in one class of apparatus it is desirable to carry out the process in a vacuum tight housing in order to obtain improved freedom from contamination.
It is known in the prior art to employ a ball chain and takeup drum for vertically lifting the crystal as it is drawn from the melt. In the known apparatus the alignment of the pulling axis, gravitationally defined by the ball chain, with the rotation axis is maintained by causing the takeup drum to travel along a lead screw, transverse to the crystal pulling rotation axis.
The known drum translation apparatus employes a keyed or splined mechanism, or linear ball bushing to constrain the drum translation to its rotational axis during rotation of the drum. A fixed nut and lead screw on the axis of the drum have such a thread pitch to translate the drum by an amount equal to the width of the chain. Thus, the chain is maintained tangent with the drum circumference and in constant alignment with the desired crystal rotation axis. The lubrication requirements for the lead screw and spline of the known chain pulling mechanism is clearly ill-suited to contain a contamination-free environment desired for crystal growth with the present invention.
The transverse travel requirement for the drum of the prior art mechanism further enlarges the volume of the pulling head and affects the balance of the mechanism which controls rotation about the vertical axis. The winding of the chain on the drum is a source of uneven lift as the chain increments relax against the drum surface while under tension. Such relaxation occurs in an uncontrolled fashion and small irregularities in vertical pulling are introduced thereby.
Crystal pulling apparatus of the form above described utilizes a chain taking the form of a linkage of spherical beads. This arrangement has a number of advantageous properties but the chain must be replaced at moderately frequent intervals of use. In the prior art one end of the chain is secured to the drum directly via mechanical fastener. Consequently the entire chain must be replaced as a routine preventative maintenance measure requiring that the pulling head chamber be opened for access.